The effects of the preponderance of drugs result from their interaction with functional macromolecular components of the organism. Such interaction alters the function of the pertinent cellular component and thereby initiates the series of biochemical and physiological changes that are characteristic of the response to the drug. The term “receptor” denotes the component of the organism with which the chemical agent interacts. There are fundamental corollaries to the statement that the receptor for a drug can be any functional macromolecular component of the organism. One is that a drug is potentially capable of altering the rate at which any bodily function proceeds; a second is that, by virtue of interactions with specific receptors, drugs do not create effects but merely modulate the rates of ongoing functions. A simple pharmacological dictum thus states that a drug cannot impart a new function to a cell. Functional changes due to a drug result from either enhancement or inhibition of the unperturbed rate. Furthermore, a drug that has no direct action can cause a functional change by competition for a binding site with another, active regulatory ligand of the receptor. Drugs are termed agonists when they cause effects as a result of direct alteration of the fundamental properties of the receptor with which they interact. Compounds that are themselves devoid of intrinsic pharmacological activity, but cause effects by inhibition of the action of a specific agonist (e.g. by competition for agonist binding sites) are designated as antagonists.
At least from a numerical standpoint, the proteins of the cell form the most important class of drug receptors. Examples include the enzymes of crucial metabolic or regulatory pathways (e.g., tyrosine hydroxylase; 3-hydroxy-3-methylglutaryl-CoA reductase). Of equal interest are proteins involved in transport processes (e.g. Ca2+-ATPase; Na+-K+-ATPase) or those that are protein kinases which activate other proteins as a consequence of their binding a secondary messenger such as cAMP. Specific binding properties of other cellular constituents can be exploited. Thus, nucleic acids are important drug receptors, particularly for chemotherapeutic approaches to the control of malignancy, and plant lectins shown remarkable specificity for recognition of specific carbohydrate residues in polysaccharides and glycoproteins. Small ions such as Ca2+ which can function as a regulatory ion or Fe2+ which can serve as an essential enzymatic cofactor can be exploited as drug receptors. And, drugs can also produce a functional change by a nonreceptor-mediated action. Certain drugs that are structural analogues of normal biological constituents may be incorporated into cellular components and thereby alter their function. This has been termed a “counterfeit incorporation mechanism” and has been implemented with analogues of purines and pyrimidines that can be incorporated into nucleic acids and that have utility in cancer chemotherapy and that have antiviral activity. Also, specific constituents of pathogens can be exploited as receptors. For example, the electron carriers of bacteria can serve as receptors as described U.S. application Ser. No. 948,326, which is incorporated herein by reference, and the replicative enzymes of viruses can be serve as receptors for the virus HIV. Many compounds are known which have receptor or nonreceptor mediated in vitro activity as appears in The Handbook of Enzyme Inhibitors, Mahendra Kumor Jain, 1982, Wiley Interscience, New York, incorporated herein by reference. However, only a small percentage produce the desired functional change in vivo or have a high therapeutic ratio, because they are toxic in their free form; they are rapidly inactivated or excreted; or, they cannot obtain access to their target receptor or site of action because they are impermeant to cells or biological barriers such as the blood brain barrier due to unfavorable energetics due, for example, to the possession of polar or charge groups; or, they are toxic as a consequence of being nonselective with regards to their access to and action with receptors in one biological environment or compartment relative to another. In these cases, compounds which demonstrate in vitro efficacy are ineffective therapeutics.